1. Statement of the Technical Field
The inventive arrangements relate generally to inductors and more particularly to toroidal inductors.
2. Description of the Related Art
As is well known, a magnetic field is generated each time an electric current is present in a conductor. An inductor is a passive electrical component that includes a series of conductive windings or coils (hereinafter “turns”) which cooperate to define the magnetic field in a specified region when an electric current is established in the turns. The ability of an inductor to store energy in the magnetic field is described by an inductance L, which is generally proportional to the square of the number of turns N2 and the permeability μ of the regions in which the magnetic field is established. The permeability μ oftentimes is discussed in terms of relative permeability μr, which is the ratio of the permeability ∞ to the permeability of free space μ0. i.e.
      μ    r    =      μ          μ      0      
Often times inductors are wound on ferromagnetic cores having a permeability which is greater than air (i.e. μr>1.0) in order to provide a greater inductance for a given number of turns. Such cores are available in a variety of shapes ranging from simple cylindrical rods to donut-shaped toroids. Toroids are known to provide certain advantages since, for a given permeability and number of turns, they provide a higher inductance as compared to solenoidal (rod-shaped) cores. Toroids also have the advantage of substantially containing the magnetic field produced by the inductor within the core region so as to limit RF leakage and minimize coupling and interference with other nearby components. For a typical toroidal inductor the inductance is given by the following equation:
  L  =                    μ        ⁢                                  ⁢                  N          2                ⁢        h                    2        ⁢                                  ⁢        π              ⁢    ln    ⁢          b      a      in which h is a height of the inductor, a is an inner radius of the inductor, and b is an outer radius of the inductor.
In miniature RF circuitry, however, implementation of toroidal inductors is particularly difficult. Accordingly, inductors in miniature RF circuitry often tend to be implemented as surface mount components or as planar spirals formed directly on the surface of an RF substrate. Planar spiral inductors suffer from a serious drawback in that, in contrast to a toroidal inductor, they do not substantially contain the magnetic field that they produce. While surface mount toroidal inductors work well, the circuit board real estate required for such components is a significant factor contributing to the overall size of RF systems. Indeed, the use of passive surface mount devices oftentimes requires a circuit board to be larger than would otherwise be necessary to contain the circuit elements.
U.S. Pat. No. 5,781,091 to Krone, et al discloses an electronic inductive device and method for manufacturing same in a rigid copper clad epoxy laminate. The process involves drilling a series of spaced holes in an epoxy laminate, etching the copper cladding entirely off the board, positioning epoxy laminate over a second laminate, positioning a toroidal ferromagnetic core within each of the spaced holes, and filling the remainder of each hole with a fiber-filled epoxy. This technique involves numerous additional processing steps that are not normally part of the conventional steps involved in forming a conventional epoxy circuit board. These additional steps naturally involve further expense. Also, such techniques are poorly suited for use with other types of substrates, such as ceramic types described below.
Glass ceramic substrates calcined at 850˜1,000C are commonly referred to as low-temperature co-fired ceramics (LTCC). This class of materials have a number of advantages that make them especially useful as substrates for RF systems. For example, low temperature 951 co-fire Green Tape™ from Dupont® is Au and Ag compatible, and it has a thermal coefficient of expansion (TCE) and relative strength that are suitable for many applications. Other LTCC ceramic tape products are available from Electro-Science Laboratories, Inc. of 416 East Church Road, King of Prussia, Pa. 19406-2625, USA. Manufacturers of LTCC products typically also offer metal pastes compatible with their LTCC products for defining metal traces and vias.
The process flow for traditional LTCC processing includes (1) cutting the green (unfired) ceramic tape from the roll, (2) removing the backing from the green tape, (3) punching holes for electrical vias, (4) filling via holes with conductor paste and screen printing patterned conductors, (5) stacking, aligning and laminating individual tape layers, (6) firing the stack to sinter powders and densify, and (7) sawing the fired ceramic into individual substrates.
LTCC processing requires that materials that are co-fired are compatible chemically and with regard to thermal coefficient of expansion (CTE). Typically, the range of commercially available LTCC materials have been fairly limited. For example, LTCC materials have been commercially available in only a limited range of permittivity values and have not generally included materials with relative permeability values greater than one. Recently, however, developments in metamaterials have begun to expand the possible range of materials that can be used with LTCC. Further, new high-permeability ceramic tape materials that are compatible with standard LTCC processes have become commercially available.